Having been Keto for nearly 2 years, I am now prepping for a photoshoot and intend to exit keto but have problems filling out my muscles.

Tried out several carbing up sessions, ranging from 500 - 1200 gms of carbs with a variety of sources (fast & slow) and duration (12 hours - 2 days) but my muscles are still flat. The higher carb up sessions resulted in a watery look but muscles still flat.

May I seek advice from experts here who many help me attain the dry and full look for my photoshoot?

I'd say you are probably keto adapted. Loading up on carbs right before a shoot isn't going to give you some positive looking, pumped up look. You just haven't had the time to adjust to the sudden change in diet.